Dec. 4, 2001 The idea that the Earth was encased in ice some 650 million years ago has sparked much scientific debate in recent years. In the ongoing Snowball Earth "fight," scientists continually uncover and report new evidence that supports their respective views. Martin Kennedy, from the University of California, Riverside, has just tossed a curveball into the Snowball Earth theory with new data he reports in the December issue of GEOLOGY.
The Snowball Earth hypothesis proposes that during several profoundly cold periods of Earth's history that occurred from 750 to 600 million years ago, the ocean was covered by a thick sheet of ice. This would have had a huge impact on early life on Earth, and would have just about wiped it out entirely. Scientists generally agree that ice sheets reached low latitudes on the planet during severe glaciation, but the idea of a completely ice-covered ocean is what's being disputed.
Kennedy and his colleagues' most recent research reveals that life in the oceans during the "snowball" event basically went on as usual. This new data is difficult to reconcile with the effects on life an entirely ice-covered ocean would have imposed, and this fundamentally challenges the Snowball hypothesis.
This new evidence also supports alternative models, such as Kennedy's post-glacial release of methane from massive Clathrate destabilization that he proposed in last May's issue of GEOLOGY.
For the last six years, Kennedy has collected limestone and dolomite rocks from Precambrian glacial deposits to establish a record of carbon isotopic variation through the glacial interval. These data indicate consistent positive isotopic values from glacial rocks in northern Namibia, central Australia, and the North American Cordillera.
"The criteria was that they had to occur in a marine succession and they should not be detrital but precipitated in situ," Kennedy said. "I also wanted to have as many examples as possible to establish a global record and discount local effects or misinterpretation from a single locality. While one interval might be lacustrine, it is unlikely that all are, and since they all show roughly the same positive values then it is more likely to be a meaningful result."
His evidence shows that the carbon isotope 13C to 12C ratio was actually higher during the glaciation indicating the presence of a healthy and productive marine ecosystem. This ratio dropped only after the ice had melted and this suggests that other influences other than those proposed in the Snowball hypothesis must have been active.
"If there was no photosynthesis or life in the ocean, the carbon isotope values would be the same as the mantle," Kennedy said. "Only the presence of life causes a difference in those values. We did not find isotopic evidence that a global ice sheet impacted overall marine productivity. We would think that if an ice sheet covered the oceans, it would have had an impact on marine production or photosynthesis and we find no carbon isotopic evidence for this. The oceans just look normal."
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